1. Field of the Invention
This invention relates to a motion processing circuit for detecting and processing motion of an image provided by a video signal, and more particularly to an edge detection circuit for detecting presence or absence of motion of an edge provided by a video signal and a motion processing circuit which includes an edge detection circuit of the type mentioned.
2. Description of the Related Art
Generally, for color television signals of the NTSC system and the PAL system, a composite video signal which includes a brightness signal and a color signal for each frame is used. Further, for such a composite video signal as just mentioned, a technique of motion detection is used in order to efficiently effect decomposition of a brightness signal and a color signal (such decomposition will be hereinafter referred to as YC decomposition), noise reduction, double speed interpolation and signal compression processing.
According to the technique of motion detection, a variation (finite difference) between a current frame and a preceding frame is detected, and, based on presence or absence of such variation, a motion signal representative of whether or not there is motion between the frames is produced. By this technique, the adaptability of processing such as YC decomposition or signal compression processing mentioned above can be improved.
For example, in YC decomposition or double speed interpolation processing of the NTSC system, where an image exhibits a stationary state, a complete YC decomposition signal or interpolation signal can be obtained by inter-frame calculation and an image of a high quality can be obtained. On the other hand, in signal compression processing, where an image exhibits a stationary state, a signal of a preceding frame can be used as it is for a current frame, and consequently, the amount of signals to be transmitted can be reduced and the transmission band of the transmission signal can be made narrow. Accordingly, the technique of motion detection is a very important technique in video signal processing.
On the other hand, it is pointed out that, at an edge portion representative of a contour of a pattern on a screen, an error is likely to occur in detection of motion described above. Such erroneous detection is caused by crosstalk between a high brightness portion included in an edge component and a color signal portion, very small motion of a high brightness portion included in an edge component, and so forth. Of those factors, the crosstalk between a high brightness portion and a color signal portion arises from the fact that, while the brightness signal has a same phase between different frames, the color signal has opposite phases to each other between adjacent frames.
Since an edge portion includes crosstalk and very small motion in this manner, a motion adaptive YC decomposition circuit or a motion adaptive double speed conversion circuit which ignores an error upon motion detection at an edge portion has a drawback in that dot disturbance, cross color disturbance, fading of an image and so forth occur at an edge portion provided by a video signal, resulting in reproduction of an unbecoming image. Therefore, it is desirable to prevent erroneous detection of motion arising from very small motion at an edge portion.
A motion information signal detection circuit as a kind of motion processing circuit which prevents erroneous detection of motion at an edge portion is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 3-220889. The motion information signal detection circuit disclosed in the document Just mentioned includes a motion detection circuit for detecting motion between frames from a video signal and outputting a motion signal, an edge detection circuit for discriminating an edge portion of a current frame and generating, when an edge portion is detected, an edge detection signal, and an adaptive processing circuit which is controlled in response to the edge detection signal. The edge detection circuit generates, if it detects an edge portion, an edge detection signal for a period of time corresponding to the detected edge portion, and forwards the edge detection signal as a masking signal to the adaptive processing circuit. Upon reception of the edge detection signal, the adaptive processing circuit attenuates the level of the motion signal from the motion detection circuit in accordance with the level of the edge detection signal.
Accordingly, the motion information signal detection circuit described above can eliminate erroneous detection of motion at an edge portion by either decreasing the level of or masking the motion signal at an edge portion in accordance with the level of the edge detection signal. This signifies that the motion signal is either decreased or substantially held from being outputted for a period of time of the edge portion provided for the duration of the edge detection signal, and consequently, very small motion between frames can be ignored. In other words, the conventional motion information signal detection circuit can prevent erroneous detection arising from very small motion at a stationary portion of a pattern.
However, where the construction of the motion information signal detection circuit described above is employed, also when an edge in a current frame exhibits a variation from that in the preceding frame over a time longer than the duration of the edge detection signal, that is, also when an edge portion actually exhibits motion, the edge detection signal is forwarded for each frame to the adaptive processing circuit. Consequently, although there actually is some motion between frames, an edge detection signal is forwarded to the adaptive processing circuit for each frame similarly as in the case wherein an image exhibits a stationary state, and an output for the edge portion of each frame is decreased or masked. Accordingly, although an edge portion exhibits motion between frames, the motion of the edge portion is ignored. This represents that, with the construction of the motion information signal detection circuit described above, such erroneous detection that motion is detected as a stationary state occurs.
Also it is possible to reduce erroneous detection at an edge portion to some degree by increasing, without using an edge detection circuit, the number of frames to be used for motion detection. However, as such number of frames increases, the number of frame memories, which are expensive, must be increased, which is not preferable for economy.